It is a common practice to make teeth of lifters perpendicular to the wall of the autogenous mill. Yet for such an installation, as the mill turns, the direction of velocity of falling rocks relative to teeth of lifters will be inclined to the direction of the latter (FIG. 2). The intermittent side impact of rocks on teeth of lifters causes the latters to vibrate, and also causes uneven wear pattern of the lifters. For the above reason, for example, lifter bars of Allis-Chalmers Rockcyl autogenous mills are forced to be designed for easy removal, reversal and replacement, and the new Rockcyl primary autogenous mills have to reverse the direction of rotation of mills on time to achieve balanced lifter and liner life.
In addition to the above inconveniences, the rock holding ability of such lifters is comparatively low. The mill is then forced to run faster (up to 83% to 87% of its critical speed) for greater centrifugal force so as to maintain reasonable lifting of rocks. Under the influence of such centrifugal force, fine particles sticking on the wall of the mill will turn round in it (FIG. 3), and will be difficult to be drawn out of the mill by the suction pipe located at the center of the mill. As a result, the efficiency of the mill drops down, and the percentage of undesired fines in the final product increases.